Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Selective Catalytic Reduction (SCR) Catalyst
The SCR catalyst reduces nitrogen oxides (NOx) present in the exhaust stream to nitrogen (N2 ) and water (H2 O). The SCR catalyst contains a copper catalyst washcoated on a zeolite substrate. At the inlet of the SCR catalyst is a port for the reductant injector, followed by a louvered diffuser and a twist mixer. The reductant diesel exhaust fluid (DEF) is a solution of urea in deionized water. The urea solution percentage for correct SCR system operation is 28 - 35%. When DEF is introduced into the system, it finely atomizes in the louvered diffuser and mixes evenly with exhaust gases in the twist mixer. During this time, the heat of the exhaust gases causes the urea to split into carbon dioxide (CO2 ) and ammonia (NH3 ). As the ammonia and NOx pass over the SCR catalyst, a reduction reaction takes place and the ammonia and NOx are converted to N2 and H2 O. This reaction takes place at up to 95% efficiency and allows the engine to run leaner and more efficiently, since the high NOx levels that are produced under lean conditions are eliminated.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.